The field of the present invention is that of turbojets and members providing the connection between the various parts thereof. The invention relates more particularly to members providing the mechanical support of the bypass flow duct of bypass engines.
A propulsion engine, such as a turbojet, may be mounted in various locations of the aircraft, by fastening onto a mast or a pylon belonging to the structure thereof. It may also be suspended below the wings, fixed to the fuselage, generally at the rear, or mounted in the tail unit by appropriate fastening means. Said fastening means have the function of ensuring the transmission of mechanical forces between the engine and the structure of the aircraft. The loads to be taken into account are, in particular, the weight of the engine along the vertical Z-axis, its thrust along the X-axis of the engine and the lateral aerodynamic loads along the transverse Y-axis. The loads to be transmitted also include the torque pick-up about the shaft of the engine. Said means also have to absorb the deformations to which the engine is subjected during the different phases of the flight which result, for example, from dimensional variations due to thermal expansion or contraction, without transmitting said deformations to the mast.
A method of suspension, for example in the case of a turbo-fan engine, consists in fastening the engine to a mast belonging to the structure of the wing of the aircraft by a front suspension or front fixing and a rear suspension or rear fixing. The front suspension is fixed, in particular, to the intermediate casing downstream of the fan casing and the rear suspension to the exhaust casing of the primary flow. Said two parts constitute the structural elements of a turbo-engine, on which all the forces are absorbed.
Modern turbojets are bypass turbo-engines with a high by-pass ratio, the secondary air flow being compressed by a single compressor stage known as the fan. At the outlet of this stage the air flow is guided by a duct directly into a nozzle to contribute to the thrust of the engine. It circulates, therefore, between the principal body of the engine defined by the casings and a bypass flow duct (generally denoted by the English acronym OFD for Outer Fan Duct). For reasons of mass, said duct is commonly made from composite material. The fixing of said duct to the engine is carried out by connections at its two longitudinal ends, a first fixing being carried out upstream on the casing surrounding the fan and a second fixing to the rear on a support ring carried by the exhaust casing.
The connection between the support ring and said exhaust casing is generally carried out by means of arms which pass through the bypass flow. In more recent embodiments, said connection may also be provided by profiled connecting rods, fixed, on the one hand, to the bypass flow duct and, on the other hand, to the exhaust casing, which permits a significant reduction in the mass of this connection. In this case of carrying out the fixing by connecting rods, the connection is made by a framework of connecting rods, which are generally six or eight in number and aligned in pairs, and which are attached at three or four points to the support ring, as shown in FIG. 2.
Such an assembly is intentionally hyperstatic, in particular to ensure redundancy in the case of rupture of one of said elements. The drawback with such a choice is that the system is difficult to mount and it is necessary to provide devices for adjusting the length of the connecting rods, without which the mounting becomes impossible. Thus it is necessary to provide a system for adjusting the position of the ball joints as a function of the length of the connecting rods, to take into account the manufacturing tolerances of each of the connecting rods and clevises of the exhaust casing and, if required, to provide them with pre-stressing during the mounting.